The present invention relates to an MOS semiconductor device with a semiconductor substrate of a first conductivity type having a high impurity concentration for preventing punch-through between the source and drain regions.
An MOS semiconductor device such as an n-channel MOS transistor comprises a p-type monocrystalline silicon substrate, an element isolation region (a field oxide film) formed on the main surface of the substrate to form a plurality of islands in the substrate, n.sup.+ -type source and drain regions formed in the main surface of this island substrate region to be mutually isolated, a gate oxide film formed on the surface of the channel region between the source and drain regions, and a gate electrode formed on the gate oxide film to oppose the channel region therethrough.
With advances in the micropatterning of LSIs using this type of MOS semiconductor, reduced channel lengths have brought about increases in current drive capacity and LSI switching speeds. As channel length is reduced, however, the source and drain regions become closer together, resulting in punch-through in the area therebetween.
As a result, a conventional n-channel MOS transistor like the one shown in FIG. 1 has been developed. This transistor uses a high impurity concentration monocrystalline silicon substrate to prevent punch-through. In FIG. 1, reference numeral 11 denotes a p.sup.+ - type monocrystalline silicon substrate. Element isolation region (field oxide film) 12 is formed on the main surface of substrate 11 to divide the surface of substrate 11 into a plurality of islands. N.sup.+ -type source and drain regions 13 and 14 are formed to be mutually separated in the main surface of the island substrate region. The surface of the substrate between source and drain regions 13 and 14 serves as channel region 11b. Oxide film 16 is formed on the surface of region 11b. Gate electrode 15 is then formed on the film 16 to oppose region 11b therethrough. Since substrate 11 in an MOS transistor having this type of structure has a high impurity concentration, it is possible to control spreading of a depletion layer between channel region 11b and source and drain regions 13 and 14. Hence, punch-through can be prevented. Giving substrate 11 a high impurity concentration, however, also results in a reduced pinch-off voltage (to be described later) and makes it impossible to obtain a sufficiently large current flow between regions 13 and 14.
More specifically, assume that voltages of 0 V, 50 mV, and 5 V are applied to source region 13, drain region 14, and gate electrode 15, respectively, in the MOS transistor shown in FIG. 1. Inversion layer 17 is induced in the surface of the channel region, and a current flows through this layer from region 13 to region 14. Depletion layer 18 from electrode 15 to the channel region, shown by the broken line, extends from regions 13 and 14 in the direction of the channel region and substrate 11 as shown by the broken lines to become depletion layers 19 and 20. A negative fixed charge exists within depletion layers 18 to 20. Given these conditions, if the voltage being applied to region 14 is increased to 5 V in order to create a pentode operation region, the depletion layer extends as shown by the broken line in FIG. 2. Here, as opposed to the case of FIG. 1, inversion layer 17 does not appear evenly throughout the surface of region 11b but instead depletes around region 14. This boundary (shown in the diagram by reference numeral 10) is called the pinch-off point. Likewise, a region which has depleted as far as the surface is called a pinch-off region, and the drain voltage that occurs in the pinch-off region is called the pinch-off voltage. When the gate voltage is constant, the drain current increases along with the increase in drain voltage. If the gate voltage becomes larger than the pinch-off voltage, however, the drain current is saturated and becomes constant. Pinch-off point 10 has a potential somewhere between the voltages applied to the source and drain regions (from 0 to 5 V), and p.sup.+ -type substrate 11 is grounded. This has the same effect as applying a back bias to substrate 11, and the channel region becomes increasingly difficult to invert. As a result, creation of a pinch-off region becomes easy.